LIVERMORE, Calif. — The first experiments at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) have demonstrated a unique physics effect that bodes well for NIF’s success in generating a self-sustaining nuclear fusion reaction.
NIF's DANTE diagnostic
Dante Diagnostic: Many instruments-detectors, oscilloscopes, interferometers, streak cameras, and other diagnostics-surround the target chamber to measure the system's performance and record experimental results. By characterizing the X-rays generated during NIF experiments, including the latest laser-plasma interaction experiments described in Science Express, the Dante soft X-ray power diagnostic helps scientists understand how well the experiment performed.
Click for high resolution image

In inertial confinement fusion (ICF) experiments on NIF, the energy of 192 powerful laser beams is fired into a pencil-eraser-sized cylinder called a hohlraum, which contains a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen. Rocket-like compression of the fuel capsule forces the hydrogen nuclei to combine, or fuse, releasing many times more energy than the laser energy that was required to spark the reaction. Fusion energy is what powers the sun and stars.

The interplay between NIF’s high-energy laser beams and the hot plasma in NIF fusion targets, known as laser-plasma interactions, or LPI, has long been regarded as a major challenge in ICF research because of the tendency to scatter the laser beams and dissipate their energy. But during a series of test shots using helium- and hydrogen-filled targets last fall, NIF researchers were able to use LPI effects to their advantage to adjust the energy distribution of NIF’s laser beams.

The experiments, described in an article in today’s edition of Science Express, the online version of the journal Science, resulted in highly symmetrical compression of simulated fuel capsules – a requirement for NIF to achieve its goal of fusion ignition and energy gain when ignition experiments begin later this year.

“Laser-plasma interactions are an instability, and in many cases they can surprise you,” said ICF Program Director Brian MacGowan. “However, we showed in the experiments that we could use laser-plasma interactions to transfer energy and actually control symmetry in the hohlraum. Overall, we didn’t find any pathological problem with laser-plasma interactions that would prevent us generating a hohlraum suitable for ignition.”

Using LPI effects to tune ICF laser energy is “a very elegant way to do it,” said Siegfried Glenzer, NIF plasma physics group leader. “You can change the laser wavelengths and get the power where it’s needed without increasing the power of individual beams. This way you can make maximum use of all the available laser beam energy.”
NIF hohlraum sketch
This artist's rendering shows a NIF target pellet (the white ball) inside a hohlraum capsule with laser beams entering through openings on either end. The beams compress and heat the target to the necessary conditions for nuclear fusion to occur.
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In the Science Express article, Glenzer, MacGowan and their NIF colleagues reported that “self-generated plasma-optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, producing symmetric X-ray drive.” Glenzer said the gratings act like tiny prisms, redirecting the energy of some of the laser beams just as a prism splits and redirects sunlight according to its wavelength.

Glenzer attributed the new LPI phenomenon to the size of the test hohlraums, which, while somewhat smaller than actual NIF ignition targets, are two to three times larger than hohlraums used in previous ICF experiments at other laser facilities. He said the increased amount of the high-temperature, low-density plasma in the areas where the laser beams enter the hohlraum was responsible for the spontaneous generation of the plasma gratings.

The technique of slightly shifting the wavelength of some laser beams to control the transfer of energy between the beams and equalize the laser power distribution in the hohlraum had been predicted and modeled by NIF scientists using high-fidelity three-dimensional simulations. In last fall’s experiments, an initially asymmetric target implosion with a “pancake” shape was changed to a spherical shape by the wavelength-shifting technique, validating the modeling results.

The NIF laser system began firing all 192 laser beams onto targets in June 2009. In order to characterize the X-ray drive achieved inside the target cylinders as the laser energy is ramped up, these first experiments were conducted at lower laser energies and on smaller targets than will be used for ignition experiments. These targets used cryogenically cooled gas-filled capsules that act as substitutes for the fusion fuel capsules that will be used in the ignition campaign that begins this summer.
NIF target positioner
A NIF technician checks the target positioner, which precisely centers the target inside the target chamber before each experiment, and serves as a reference to align the laser beams.
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Before the wavelength-shifting effects were tested, the only way to adjust the laser energy reaching the walls of the hohlraum, where it is converted into X-rays that heat and ablate the outer surface of the fuel capsule and cause the compression of the fuel inside the capsule, was to adjust the relative energy of the laser beams in the early stages of a shot, during preamplification.

By taking advantage of the LPI effects in the target, as the beams crossed at the entrance of the hohlraums, the scientists could make use of minute wavelength adjustments, ranging from a fraction of an angstrom to a few angstroms (an angstrom is one ten-billionth of a meter, about the size of an atom). With the LPI scheme, “you can run every beam at maximum power and have another distribution mechanism to achieve symmetry,” Glenzer said.

The test shots proved NIF’s ability to deliver sufficient energy to the hohlraum to reach the radiation temperatures – more than 3 million degrees Centigrade – needed to create the intense bath of X-rays that compress the fuel capsule. When NIF scientists extrapolate the results of the initial experiments to higher-energy shots on full-sized hohlraums, “we feel we will be able to create the necessary hohlraum conditions to drive an implosion to ignition,” said Jeff Atherton, director of NIF experiments.

At the end of the experimental campaign, the NIF lasers set a world record by firing more than one megajoule of ultraviolet energy into a hohlraum – more than 30 times the energy previously delivered to a target by any laser system.

“This accomplishment is a major milestone that demonstrates both the power and the reliability of NIF’s integrated laser system, the precision targets and the integration of the scientific diagnostics needed to begin ignition experiments,” said NIF Director Ed Moses. “NIF has shown that it can consistently deliver the energy required to conduct ignition experiments later this year.”

NIF’s next step is to move to ignition-like fuel capsules that require the fuel to be in a frozen hydrogen layer (at 425 degrees Fahrenheit below zero) inside the fuel capsule. NIF is currently being made ready to begin experiments with ignition-like fuel capsules in the summer of 2010.

NIF (lasers.llnl.gov), the world’s largest laser facility, is the first facility expected to achieve fusion ignition and energy gain in a laboratory setting. NIF is an essential part of the National Nuclear Security Administration’s Stockpile Stewardship Program, which ensures the reliability and safety of the nation’s nuclear weapons stockpile without live testing. NIF experiments will also be used to conduct astrophysics and basic science research and to develop carbon-free, limitless fusion energy.

The NIF fusion ignition experiments are part of the National Ignition Campaign (NIC). NIC is a partnership among the National Nuclear Security Administration (NNSA), Lawrence Livermore National Laboratory, Los Alamos National Laboratory, the Laboratory for Laser Energetics, General Atomics, and Sandia National Laboratories as well as many other national laboratories and universities.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory that develops science and engineering technology and provides innovative solutions to our nation's most important challenges. Lawrence Livermore National Laboratory is managed by Lawrence Livermore National Security, LLC for the U.S. Department of Energy's National Nuclear Security Administration.

LIVERMORE, Calif. -- The National Nuclear Security Administration's National Ignition Facility (NIF) has set world records for neutron yield from laser-driven fusion fuel capsules and laser energy delivered to inertial confinement fusion (ICF) targets. NIF researchers will report on these and other recent experimental results this week at the annual meeting of the American Physical Society Division of Plasma Physics in Chicago.
The neutron yield record was set on Sunday, Oct. 31, when the NIF team fired 121 kilojoules of ultraviolet laser light into a glass target filled with deuterium and tritium (DT) gas. The shot produced approximately 3 x 1014 (300 trillion) neutrons, the highest neutron yield to date by an inertial confinement fusion facility. Neutrons are produced when the nuclei of deuterium and tritium (isotopes of hydrogen) fuse, creating a helium nucleus and releasing a high-energy neutron.
On Tuesday, Nov. 2, the team fired 1.3 megajoules of ultraviolet light into a cryogenically cooled cylinder called a hohlraum containing a surrogate fusion target known as a symmetry capsule, or symcap. This was the highest-energy laser shot and was the first test of hohlraum temperature and capsule symmetry under conditions designed to produce fusion ignition and energy gain. Preliminary analysis indicated that the hohlraum absorbed nearly 90 percent of the laser power and reached a peak radiation temperature of 300 electron volts (about six million degrees Fahrenheit) -- making this the highest X-ray drive energy ever achieved in an indirect drive ignition target.
The experiments followed closely on the heels of NIF's first integrated ignition experiment on Sept. 29, which demonstrated the integration of the complex systems required for an ignition campaign including a target physics design, the laser, target fabrication, cryogenic fuel layering and target positioning, target diagnostics, control and data systems, and tritium handling and personnel and environmental protection systems. In that shot, one megajoule of ultraviolet laser energy was fired into a cryogenically layered capsule filled with a mixture of tritium, hydrogen and deuterium (THD), tailored to enable the most comprehensive physics results.
"The results of all of these experiments are extremely encouraging," said NIF Director Ed Moses, "and they give us great confidence that we will be able to achieve ignition conditions in deuterium-tritium fusion targets."
NIF, the world's largest and highest-energy laser system, is located at Lawrence Livermore National Laboratory (LLNL) in California. NIF researchers are currently conducting a series of "tuning" shots to determine the optimal target design and laser parameters for high-energy ignition experiments with fusion fuel in the coming months.
When NIF's 192 powerful lasers fire, more than one million joules of ultraviolet energy are focused into the ends of the pencil-eraser-sized hohlraum, a technique known as "indirect drive." The laser irradiation generates a uniform bath of X-rays inside the hohlraum that causes the hydrogen fuel in the target capsule to implode symmetrically, resulting in a controlled thermonuclear fusion reaction. The reaction happens so quickly, in just a few billionths of a second, that the fuel's inertia prevents it from blowing apart before fusion "burn" spreads through the capsule -- hence the term inertial confinement fusion. In ignition experiments, more energy will be released than the amount of laser energy required to initiate the reaction, a condition known as energy gain. NIF researchers expect to achieve a self-sustaining fusion burn reaction with energy gain within the next two years.
The experimental program to achieve fusion and energy gain, known as the National Ignition Campaign, is a partnership among LLNL, Los Alamos National Laboratory, the Laboratory for Laser Energetics at the University of Rochester, General Atomics of San Diego, Calif., Sandia National Laboratories, Massachusetts Institute of Technology, and other national and international partners.
In the Oct. 31 "neutron calibration" shot, NIF's lasers were fired directly onto a DT-filled glass target, as opposed to the indirect-drive geometry used in NIF's TD and THD experiments. The purpose of the shot was to calibrate and test the performance of NIF's extensive neutron diagnostic equipment.
The capsule used in the Nov. 2 symcap experiment has the same two-millimeter outer diameter doped shell as an ignition capsule, but replaces the DT fuel layer with an equivalent mass of material from the outer shell to mimic the capsule's hydrodynamic behavior. Achieving a highly symmetrical compression of the fuel capsule is a key requirement for NIF to achieve its goal of fusion ignition.
NIF, a project of the U.S. Department of Energy's National Nuclear Security Administration (NNSA) was built as a part of NNSA's program to ensure the safety, security and effectiveness of the nation's nuclear weapons stockpile without underground testing. With NIF, scientists will be able to evaluate key scientific assumptions in current computer models, obtain previously unavailable data on how materials behave at temperatures and pressures like those in the center of a star, and help validate NNSA's supercomputer simulations by comparing code predictions against observations from laboratory experiments.
Because of its groundbreaking advances in technology, NIF also has the potential to produce breakthroughs in fields beyond stockpile stewardship. NIF experiments have been conducted in support of Department of Defense X-ray testing activities, and NIF experiments in nuclear forensics and other national security areas are planned for the next several years. NIF also will provide unprecedented capabilities in fundamental science, allowing scientists to understand, for example, the makeup of stars in the universe and planets both within and outside our solar system. NIF will also help advance fusion energy technology, which could be an element of making the United States energy indepen</span>dent.

Chopstick

01-28-2011, 01:41 PM

<span style="color: #000099">The question is, if they get this thing to work, where are they going to get enough tritium to run it? The nearest abundant source is on the moon. </span>